Modern electrical therapeutic and diagnostic devices for the heart, such as pacemakers, cardiovertors, and defibrillators, for example, require a reliable electrical connection between the device and a region of the heart. Typically, a medical electrical "lead" is used for the desired electrical connection.
One type of commonly used implantable lead is a transvenous lead. Transvenous leads are positioned through the venous system to attach or electrically connect at their distal end to the heart. At their proximal end, they are connected to typically an implantable pulse generator. Such leads normally took the form of a long, generally straight, flexible, insulated conductor. Among the many advantages of a transvenous lead is that it permits an electrical contact with the heart without physically exposing the heart itself, i.e., major thoracic surgery is not required.
The specific design of a transvenous lead used is often varied depending upon the region of the heart to which it is to be connected. For example, U.S. Pat. No. 4,402,330 of Lindemans discloses a body implantable lead in which the lead body has a J-curve and the distal electrode has a permanent bend. In such a manner, the lead is configured to electrically connect to the right atrium.
While such a lead has been found acceptable for electrically connecting and thus pacing the right atrium, the need exists for a transvenous medical electrical lead which may provide an electrical connection to the left atrium or even the left ventricle. Of course the left side of the heart cannot, at present, be transvenously accessed with a lead for chronic implantation due to the direction of blood flow and the present limitations of materials. To be precise, blood flows through the right side of the heart (atrium and ventricle), through the lungs, through the left side of the heart (atrium and ventricle) and then through the rest of the body, including the brain, before returning again to the right side of the heart. Implanted objects, however, may cause minor blood clots and thrombus to form in the blood. These may, on occasion, dislodge and be released into the bloodstream. Because the blood circulates directly from the left atrium and ventricle to the brain, any clots, however minor, could have serious consequences if they were to reach the brain, e.g. a stroke. In contrast, any clots released from an object implanted in the right side of the heart would simply travel to the lungs, where they would lodge without any serious risk. Thus at present, chronic transvenous leads may not be safely implanted within the left side of the heart.
In spite of the difficulties, there remains a great need to be able to electrically stimulate or sense or both the left side of the heart. The most obvious reason is the left side of the heart accounts for the majority of the heart's hemodynamic output. For example, the left ventricle has a greater wall thickness (10-20 mm as compared to 1-5 mm) than the right side. This, of course, is reasonable given that the left side of the heart must pump blood throughout the body while the right side only pumps blood through the lungs.
Because the left side is relatively more important for hemodynamic output, not surprisingly, various pathologies may be better treated through stimulation on the left side of the heart. For example, in patients with dilated cardiomyopathy, electrical stimulation of both the right side and the left side of the heart has been shown to be of major importance to improve the patient's well-being and manage heart failure. See, for example, Cazeau et al., "Four Chamber Pacing in Dilated Cardiomyopathy," PACE, November 1994, pgs. 1974-79. See also Brecker and Fontainem,St. et al., "Effects Of Dual Chamber Pacing With Short Atrioventricular Delay In Dilated Cardiomyopathy," Lancet November 1992 Vol. 340 p1308-1312; Xiao H. B. et al., "Effect Of Left Bundle Branch Block On Diastolic Function In Dilated Cardiomyopathy," Br. Heart J 1991, 66(6) p 443-447; and Fontaine G et al, "Electrophysiology Of Pseudofunction," CI.Meere (ed.) Cardiac pacing, state of the art 1979, Pacesymp, 1979 Montreal..
At present there are several techniques for implanting a lead onto or into the left side of the heart. First, of course, is through general thoracic surgery; either via a median sternotomy; intercostal approach; or, in a more limited procedure, a sub-xiphoid approach. These procedures, however, involve major surgery which may be painful and dangerous for the patient, as well as extremely costly. The sub-xiphoid approach, moreover, only permits limited access. Another approach used to electrically access the left side of the heart is through the coronary sinus.
The coronary sinus, however, presents challenges in both implanting the lead in the proper position as well as ensuring the lead maintains sufficient electrical contact with the desired tissue. For example, coronary sinus and the related vessels are vital to the proper circulating of blood through the heart tissue. Thus, the occlusion of blood flow in the vessel by a lead should not occur. Moreover, the lead should not cause, by its presence, an extensive amount of thrombosis to form. Both the occlusion and thrombosis may hinder blood flow to the point that stagnation occurs in the vessel. The health of the surrounding tissue would be impacted. Moreover, the coronary sinus and its related vessels typically have an extremely tortuous path, especially in the more distal locations. These locations, however, are the preferred locations for providing electrical stimulation. Thus such leads must be able to follow and remain within the various tortuous locations within the coronary sinus and its related vessels. Once the lead is properly positioned it is essential that the electrode has good contact with the inside of the vessel. Further complicating matters is the fact that blood valves may be present within various portions of the coronary sinus and its related vessels. These valves may hinder the passage of the lead through the vessel. Due to the many obstacles faced by placing a lead within these vessels, it is thus preferred by physicians that the lead be able to be placed using a guide wire.
It is thus an object of the present invention to provide a medical electrical lead which may be positioned within the more distal tortuous portions of the coronary sinus and its related vessels.
A further object of the present invention is to provide such a lead which will not occlude blood flow through the vessel in which it is placed.
It is a further object of the present invention to provide a lead having an electrode which will maintain good contact with the inside of the vessel wall and thereby provide excellent electrical properties.
It is a still further object of the present invention is to provide a medical electrical lead which may be positioned along a selected portion of the coronary sinus wall placed using a guide wire.
It is a still further object of the present invention is to provide a medical electrical lead having an electrode which may be easily positioned along a selected portion of the coronary sinus wall through rotation of the electrode portion, but which may be as easily removed from the coronary sinus through counter rotation of the electrode portion.